1. Field of the Invention
The present invention relates to a sheet processing apparatus having sheet stacking means for stacking discharged sheets in succession, and to an image forming apparatus provided with such sheet processing apparatus.
2. Related Background Art
Among the sheet processing apparatus, there is already known a configuration in which the top surface of the sheets stacked on the stacking tray always remains at a predetermined height. In such configuration, there are provided a sheet stacking tray on which the discharged sheets are stacked in succession, elevate/lower means for elevating or lowering the tray, a top surface sensor for detecting the top surface of the uppermost one of the sheets stacked on the tray, and control means for controlling the elevate/lower means according to the result of detection. Such top surface sensor is comprised, for example, of a transmissive photosensor having a light emitting unit and a light receiving unit respectively at the left and right sides of the sheet stacking tray in such a manner that the optical axis of the sensor crosses transversally above the sheet stacking tray.
As the discharged sheets are stacked in succession on the stacking tray, the position of the uppermost sheet becomes higher. The optical axis is intercepted by the uppermost sheet when it reaches the optical axis, whereby the sensor detects the uppermost sheet. Based on the result of such detection, the control means controls the elevate/lower means to lower the stacking tray. The stacking trays descends by an amount allowing the intercepted optical axis is restored. The above-described operation is repeated for every sheet stacking on the stacking tray whereby the height from the sheet discharging outlet to the uppermost stacked sheet, or the dropping height of the sheet at discharge, can be maintained constant. Thus, satisfactory discharge and stacking of sheets can be ensured.
However, the above-described conventional configuration functions satisfactorily when the discharged sheets are stacked in a satisfactorily flat state on the stacking sheet, but the following drawbacks are encountered in cases where the discharged sheet is stacked in a partly lifted state.
More specifically, in the discharging and stacking of a sheet on the preceding sheets which are already stacked in a satisfactorily flat state, if the discharged sheet is stacked in a partially lifted state floating from the preceding sheets, for example if the front end of the discharged sheet does not reach the front end of the preceding stacked sheets and the rear end of the discharged sheet is lifted up, leaning on the lateral wall of the sheet processing apparatus (hereinafter such lifted or floating portion being called "deformed portion"), the top surface sensor may detect such a deformed portion. In such a case, the stacking tray is lowered until the deformed portion completely clears the top surface sensor, so that the dropping height for the next sheet becomes larger than in the ordinary state, and the next sheet cannot be ensured of satisfactory discharge and stacking. Also, if the deformed portion of the sheet is not resolved by the descent of the stacking tray, the deformed portion is folded by the sheet discharged and stacked next.